Lithography involves the transfer of a specific pattern onto a surface, and can be used to transfer a variety of patterns onto many types of materials. In particular, lithography is a process currently used to make integrated circuits (“ICs”) or computer chips. FIG. 9 is a block diagram of an example lithography system 900 for use in manufacturing computer chips under the prior art. The lithography process defines a pattern specifying the location of electrical circuits of the computer chips like metal, insulators, doped regions, and other features of a circuit printed on a silicon substrate (also referred to as a “wafer”) or other substrate. This pattern or stencil is also referred to as a mask. Computer chip manufacturers currently produce ICs by shining light 910 from an illumination source 902 through the mask 906 of the circuit design. The light is then passed through a system of optical lenses 908 to reduce the size of the image, and projected onto a wafer 972 covered by a photosensitive resist 974, also referred to as photoresist 974. The system of optical lenses 908 properly collimates the light for better feature resolution.
The mask 906 allows incident light 910 to pass through areas that define the circuit features but not through other areas. In a manner analogous to that in which light projected through a photographic negative exposes photographic paper, light coming through the mask 906 exposes the photoresist 974. Following transfer of the mask onto the photoresist 974, chemicals are used to wash away the exposed areas of the photoresist 974, leaving the pattern of the mask on the wafer. The exposed photoresist bearing the pattern of the mask selectively resists a further process (e.g., etching with acid, bombardment with various particles, deposition of a metallic or other layer, etc.). Thus, lithography can be used to effectively translate the pattern of circuit features defined by the mask 906 into a structural pattern on the wafer 972. By repeating this technique several times on the same wafer using different masks, it is possible to build multi-layered semiconductor structures and associated interconnecting electrical circuits.
Improvements in lithography have played a part in the explosive growth of the semiconductor industry and likewise the computer industry. One major improvement in lithography includes improvements in resolution resulting in a decrease in the minimum feature size (circuit features) that can be transferred to the wafer. This improvement allows for an increase in the number of transistors that can be placed on a single chip (and in the speed at which these transistors can operate). However, in spite of improvements, problems remain with lithography in the area of feature resolution on the wafer. While typical lithography techniques that use multiple exposures of multiple masks allow for an increase in circuit feature resolution, the small feature size of the circuits being transferred to the wafer makes proper alignment of the multiple masks difficult. Consequently, there is a need for lithography techniques that provide improved feature resolution without the use of multiple masks.